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voices in the ag sector hail the benefits of rising atmospheric
carbon dioxide—higher yielding, more vigorous
crops—as an unexpected boon of the industrial revolution.
This is because most plants evolved back when atmospheric
CO2 levels were quadruple what they are today. Carbon levels
tumbled when the ice ages kicked in, and most green plants
have operated at half throttle ever since.
Studies show that wheat, rice and soybeans—not to mention
many vegetable crops—could benefit from higher levels
of CO2. For starters, these crops could grow more quickly,
produce higher yields, and become inherently more drought-resistant.
But what about the weeds? Will they sit back and watch while
agriculture’s big boys take over?
Compare the adaptability of weeds and crops, and crops aren’t
even in the running, says weed ecologist
Lew Ziska of the United States Department of Agriculture’s
Agricultural Research Station in Beltsville, Maryland. It’s
weeds that thrive under widely ranging and changing conditions.
In fact, in tomorrow’s world, some agricultural weeds
may prove more difficult and costly to control using today’s
two most relied-on means—herbicide sprays and cultivation.
Since spraying herbicides outstrips all other short-term
means of dealing with weeds today, Ziska has looked at how
some of our nastiest weeds respond to herbicide regimes at
future CO2 levels. He homed in on Monsanto’s Roundup
for the simple reason that this herbicide, as well as companion
Roundup Ready crops, account for such a large share of the
agricultural marketplace.
Built-in resistance, all on their own
Roundup Ready soybeans and corn are tweaked at the core of
their being, their DNA, to provide built-in resistance to
glyphosate, the essential ingredient in Roundup. This means
conventional growers can spray their soybeans and corn after
both the crops—and many of the weeds in the neighborhood—have
sprouted. The crops live. The weeds die.
Or at least they did.
In just the past decade, some weeds have become glyphosate-resistant
all on their own. Meanwhile, other more-invasive weeds may
have become more competitive with crops. Coincidentally perhaps,
atmospheric CO2 has begun increasing at ever-accelerating
rates.
And coincidence it may be. But Ziska’s research suggests
that in some cases the connection might be causal. He has
tested how some of North American’s most notorious annual
and perennial weeds might respond to glyphosate at the CO2
levels anticipated planet-wide by 2100. He has looked at the
larger context, too, to get a feel for whether rising atmospheric
CO2 gives invasive weeds their edge.
To understand the different ways these weeds have responded,
it helps to understand what botanists mean when they toss
around phrases like “C3” and “C4”
plants. These labels tell how certain types of plants assimilate
carbon in photosynthesis: C3 plants form a pair of three carbon-atom
molecules, and C4 plants form four carbon-atom molecules.
Still poking along
"Back in the day, before weed
scientists took a closer look, it made perfect sense:
Overall, crops will do better, not worse, in the war
with weeds. When you dig deeper, though, you notice
that while farmers grow about 45 major crops, 410 weeds
reduce yields in these crops. That’s a lot of
weeds." |
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C3 plants are the ones that, since carbon levels plummeted,
are still managing at half throttle. They get stoked when
CO2 goes up. C4 plants—there’s only a handful
of them, about 4 percent of all plants—have evolved
ways to concentrate CO2. Being well-adapted to today’s
CO2 levels tends to give them a competitive edge with crops.
By and large, they don’t get much of a boost off higher
CO2.
The Greening Earth Society—an industry-backed think-tank
that questions the relationship between CO2 and global warming—will
be the first to tell you that most crops are C3 plants and
most weeds are C4 plants, and they have a point. Back in the
day, before weed scientists took a closer look, it made perfect
sense: Overall, crops will do better, not worse, in the war
with weeds. And early research bore out those assumptions.
When you dig deeper, though, you notice that while farmers
grow about 45 major crops, 410 weeds reduce yields in these
crops. That’s a lot of weeds, a big enough number that
C3 plants don’t need to be in the majority to outclass
the competition, particularly when you consider who’s
on the team. What’s your worst weed nightmare? Canada
thistle or bindweed, perhaps? Star thistle or cheatgrass?
Quackgrass, lambsquarter, spotted knapweed? C3s all.
Weeds don’t share well
CO2 is one of four critical resources plants need. Water,
sunlight, and nutrients are the others.
What if you change the amount of water a plant gets by 25
percent, Ziska asks. Will it change its growth? What about
25 percent more or less sunlight or nutrients? Will there
be winners, losers? Well, yes.
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"The thing is, we’ve had
enough experience with many plants to predict if they’ll
do better or worse with more or less sunlight or water
or nutrients. We haven’t had the same depth of experience
with changing CO2 levels." |
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Ditto with carbon dioxide, which has increased about 25 percent
since the beginning of the industrial revolution. The thing
is, we’ve had enough experience with many plants to
predict if they’ll do better or worse with more or less
sunlight or water or nutrients. We haven’t had the same
depth of experience with changing CO2 levels. We can’t
predict ahead of time just how each species will respond—or
how those physiological responses may change as each species
acclimates to rising CO2. But we can look back to research
done decades ago to help us understand the relative differences
between crops and weeds.
It was the 1950s, and fertilizer was cheap. Scientists decided
to see if dosing fields with extra nitrogen would reduce competition
from weeds. Their premise: crops and weeds that got their
fill of nitrogen would be relieved of competing for the same
limited resource, and yields would go up even if weed densities
stayed the same. Share the wealth, in other words. But weed
densities didn’t stay the same. The weeds proved better
able to use the extra nitrogen, leaving less available for
crops. Yields took a nosedive.
“Companion plants”
One of Ziska’s first projects, inspired by that nitrogen
research, looked at how field-grown soybeans, a C3 crop, would
do at ambient (today’s) and elevated (tomorrow’s)
CO2 levels with lambsquarter (a C3), and redroot pigweed (a
C4)—two of North America’s most problematic annual
weeds—along for the ride. “Companion plants,”
you might call them.
Ziska set up several “open-top chambers” in a
field at the research station. The chambers were set up so
he could add extra CO2, following the daytime-nighttime cycles
CO2 naturally travels. Some chambers got elevated CO2. Others
didn’t.
Ziska seeded soybeans in early June. Lambsquarter or pigweed
seedlings, planted in greenhouse flats at elevated or ambient
CO2 on that same day, were transplanted after the soybeans
emerged and spaced at two weeds per meter of row. All other
weeds were removed every week. And, of course, he had a control:
chambers of soybeans kept entirely weed-free at either ambient
or elevated CO2.
The soybeans in the elevated, weed-free control did great,
increasing seed yield by 23 percent and biomass by 32 percent.
But those “companion plants” took a chunk out
of yields.
C4 pigweed reduced soybean yields at ambient CO2 by 45 percent.
At elevated CO2, it reduced yields by only 30 percent: much
less, but still ahead of weed-free soybeans’ 23-percent
yield bump.
And while C3 lambsquarter reduced yields by a mere 28 percent
at ambient, at elevated levels soybean yields fell by 39 percent.
In a related project, Ziska looked at how greenhouse-grown
pigweed and lambsquarter behaved if you zap them with glysphosate
at today’s CO2 levels and those anticipated by the end
of this century.
Predictably, ramping up CO2 did nothing for pigweed survival
after the herbicide hit. It died in every replication. The
same with seedling lambsquarter. Yet 4-week-old lambsquarter
pulled back from the brink, if barely, putting out new growth—and
even flowering and setting seed—after getting Round
Up-ed at the higher seed level.
Bad weather doubles research yield
How might soybeans behave when you’re not trying to
set them up, but just want to watch how they grow with today’s
conventional protocols, only at tomorrow’s CO2 levels?
Ziska did this for two years running. The first year—2003,
the wettest in Maryland since record keeping began in 1895—the
only weeds were shallow-rooted C4 grasses favored by heavy
rains and standing water. Elevated CO2 had no effect on weed
biomass at maturity, and glyphosate provided 100-percent control.
But in 2004, with near-normal rainfall, a range of C3 and
C4 weeds popped up, including C3s lambsquarter, velvetleaf
and Virginia copperleaf, along with C4 pigweed and those C4
grasses. The elevated CO2 plots produced C3 broadleaves with
five times the biomass of those in the ambient plots. Even
after a hit of herbicide, about 6 percent of these C3s survived.
It’s in the marriage of such changed conditions and
survival rates that the potential for genetic resistance is
born. 
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